This invention relates to a control circuit utilizing silicon controlled rectifiers (SCR's) for controlling the operation of a series motor powered from a direct current source.
It is well known that the direct current supplied to a load, such as a direct current motor, from a power source, such as a battery, may be selectively varied by controlling the average power delivered to the load, and that a solid state SCR can be used as a switching device to repeatedly connect and disconnect the battery to and from the load. The power supplied to the load is determined by the ratio between the time the SCR is turned on and is conducting and the time the SCR is turned off and is non-conducting.
Turning the SCR repeatedly on and off will allow a series of pulses of current to flow through the load, the frequency of pulses being determined by the number of times the SCR is turned on per unit time and the duration, or width, of the pulses being determined by the length of time that the SCR remains on before it is turned off. If the pulse width remains constant, as it does in the present invention, the ratio of on-time to off-time will vary directly as the frequency of the pulses. As the pulse frequency increases, the off-time between the time the SCR is turned off and the time it is turned back on will decrease, and more power will be delivered to the load.
Typically, SCR control systems include a main SCR which is connected in series with the load, and a pulse generator which repeatedly supplies gate pulses to turn the main SCR on, at a rate controlled by the operator. A commutating capacitor is provided which will charge in a commutating direction through a charging SCR. The charging of the commutating capacitor is typically done when the main SCR is conducting. At the appropriate time, a commutating SCR is turned on to connect the charged capacitor across the main SCR and divert the current therefrom so that the main SCR is reversely biased and will turn off.
Several problems have arisen in the use of the SCR motor controls when used for series-connected direct-current motors used to propel vehicles, such as fork-lift trucks.
First, in starting up, bounce or erratic switch closure noise may cause erratic application of power to the motor through the main SCR.
Secondly, in vehicle controls the main oscillator which delivers gate pulses to turn the main SCR on is usually controlled by a foot-operated accelerator pedal which actuates a variable resistor, coupled to the pedal, to vary the frequency of the oscillator in response to changes in resistance of the resistor. Smoother motor control, maximum operator comfort and greatest efficiency of operation will occur if the resistance is gradually and smoothly increased on acceleration or decreased on deceleration. However, many operators do not operate vehicles in that way. They will abruptly depress the accelerator when they wish additional speed or will abruptly release the pedal when they wish to decelerate.
Thirdly, in the operation of vehicles, and particularly fork-lift trucks, the direction of the vehicle may be repeatedly reversed by switching the direction that the current flows through the field winding of the motor. If the main SCR continues to deliver current to the motor during such directional changes the contacts which connect the field winding to the armature for one direction will break current upon opening and power will be applied to the motor immediately upon closure of the contacts for establishing the reverse condition causing the vehicle to lurch.